1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material, in particular, to a thermoelectric material which is adapted to be used for a Peltier cooling element, an electric generating element, or the like, and which exhibits the Peltier effect in which a temperature difference is generated by using an electric energy or exhibits the Seebeck effect in which an electric energy is generated by using a temperature difference.
2. Description of Related Art
Thermoelectric performance of such a thermoelectric material which exhibits the Peltier effect or the Seebeck effect is evaluated by the figure of merit Z (dimension: K.sup.-1) or power factor (W/mK.sup.2) which is estimated by the following equations. The larger the value of the figure of merit Z becomes, the better the thermoelectric performance is. EQU Z=.alpha..sup.2 .sigma./.kappa. EQU Power factor=.alpha..sup.2 .sigma.
where, .alpha. is a Seebeck coefficient (.mu.V/K), .sigma. is an electrical conductivity ((.OMEGA.m).sup.-1), and .kappa. is a thermal conductivity (W/mK). Therefore, in order to obtain a thermoelectric material having a high thermoelectric performance, it is necessary to select a material having a large Seebeck-Coefficient .alpha., a large electrical conductivity .sigma., and a small thermal conductivity .kappa..
Generally, a metal material or a semiconductor material is known as a thermoelectric material. Because in a metal material, Wiedemann Franz's law holds, which says that the ratio of the electrical conductivity .sigma. to the thermal conductivity .kappa. at a temperature does not depend upon the kind of metal and therefore has a constant value in any kind of metal, there is little chance of obtaining a thermoelectric material having a high thermoelectric performance by selecting a specific kind of metal.
On the contrary, in semiconductor materials, the above described law does not necessarily hold, and it is possible to select a specific material having a large electrical conductivity .sigma. and a small thermal conductivity .kappa.. Because the value of the Seebeck coefficient .alpha. of a semiconductor material is usually about ten to several hundreds times that of a metal material, there is a good chance of obtaining a thermoelectric material having a high thermoelectric performance. Therefore, various kinds of semiconductor materials have been developed as thermoelectric materials.
Transition silicide which is used for thermoelectric power generation at a high temperature, and chalcogenide which is used as a material for Peltier cooling, are representative thermoelectric semiconductor materials which were conventionally developed. In these materials, a chalcogenide material of Bi.sub.2 Te.sub.3 system, e.g., Bi.sub.2 Te.sub.3, Sb.sub.2 Te.sub.3, PbTe, GeTe or the like, which is applied for a thermoelectric cooling device, usually presents the best thermoelectric performance near room temperature and has a high figure of merit Z more than 10.sup.-3 K.sup.-1.
However, thermoelectric performance of the above chalcogenide material of Bi.sub.2 Te.sub.3 system is extremely deteriorated in lower and higher temperature ranges other than room temperature. In particular, because the value of the figure of merit Z is not more than about 10.sup.-4 K.sup.-1 at a temperature more than about 250.degree. C. and oxidation and decomposition occur under a high temperature, it was impossible to use the above chalcogenide material in a wide temperature range.
Generally, semiconductor material other than the chalcogenide material of Bi.sub.2 Te.sub.3 system could be used as a thermoelectric material in only a very narrow temperature range.